fix(tests): sync-integration cleanup wiped the shared reading_history table

sync_integration_test's cleanup ran DELETE FROM reading_history WHERE
true — harmless for years while the table sat empty, and a silent
stats-data destroyer the moment session capture started feeding it:
every suite run erased the shared dev accounts' captured reading
(the user's manual flip-through test data included).

The cleanup is now scoped: the sync tests' sessions belong to their
test-sync users, and deleting those users cascades their rows. Any
future table-wide cleanup DELETE is off the table (AGENTS.md test
rules updated).

AGENTS.md

@@ -53,7 +53,10 @@ were name-only across ALL libraries. Fixed; do not regress.
   broke on 2026-10-14 within hours of session capture shipping).
   Tests that need deterministic data use `setup.RegularToken` +
   `getRegularUserID` (the fresh per-test user), or assert ranges
-  instead of equals.
+  instead of equals. Related: **no table-wide DELETEs in test
+  cleanups** — sync_integration's `DELETE FROM reading_history WHERE
+  true` destroyed the live reader's stats data on every suite run
+  until scoped to the test-sync users (cascade via user delete).
 
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